High spectral energy density all-fiber nanosecond pulsed 1.7 μm light source for photoacoustic microscopy.

We present a high spectral energy density all-fiber nanosecond pulsed 1.7 μm light source specifically designed for photoacoustic microscopy (PAM). The system targets the 1st overtone absorption of C-H bonds near 1720 nm within the near-infrared-III (NIR-III) window, where lipids exhibit strong optical absorption, and tissues benefit from reduced scattering and high permissible fluence. To achieve narrow-linewidth, high pulse energy, and high pulse repetition rate (PRR), we developed a master oscillator fiber amplifier architecture based on stimulated Raman scattering. A 1589.80 nm Raman pump and a custom-built narrow-linewidth Raman seed laser were employed to generate spectrally pure 1719.44 nm pulses (∼0.10 nm linewidth). The proposed light source delivers nanosecond pulses (∼5 ns) with high pulse energy (≥2.2 μJ) and tunable PRRs up to 300 kHz, resulting in a spectral energy density of approximately 22 μJ/nm-significantly higher than that of conventional 1.7 μm light sources. Performance of the NIR-PAM system was validated through resolution testing with a 1951 USAF target, demonstrating a spatial resolution of approximately 4.14 μm and an axial resolution of approximately 85.5 μm. Phantom imaging of CH-rich polymer films and ex vivo lipid-rich biological tissues confirmed the system's high spatial fidelity and strong contrast for lipid-specific structures. This compact, stable, and spectrally refined light source with high spectral energy density can offer an effective solution for high-resolution, label-free molecular imaging and represents a promising platform for clinical photoacoustic imaging applications involving lipid detection and metabolic disease diagnostics.